The transmission of force from the diesel engine to the wheels and working devices in earth-moving machines, for example dredgers or cranes, is performed by what are known as hydrostatic drives. In swash plate pumps which are often used as the drive, the torque from the crank shaft is transmitted via a swash plate with a variable setting angle to reciprocating pistons. The pistons are positioned in a rotating cylinder housing. A sliding connection to the stationary swash plate by means of a sliding shoe, commonly known in the art as a slipper, effects a reciprocating movement of the pistons with each revolution. The stroke is determined by the respective setting angle of the swash plate. The pistons which are driven in this way are responsible for building up pressure in pressurized oil lines, via which in turn a hydraulic motor can be driven.
Whereas the diesel engine generally operates in nominal load mode, the pressure which is transmitted can be continuously varied within certain limits by setting an inclined position of the swash plate and by means of the altered reciprocating strokes on the part of the pistons which result.
In drive mechanisms, the primary objective is to increase the power density. This also increases the demands imposed on the strength and emergency running properties of the slippers, since the power has to be applied at higher rotational speeds and higher pressures. Increasing the pressures increases the specific loads and consequently the stresses on the sliding surfaces. On account of the increase in the centrifugal forces associated with higher rotational speeds, all the rotating components have to be of more lightweight design. This can be achieved, for example, by minimizing the component size.
The demands imposed with regard to increasing the load-bearing capacity at the slipper are primarily satisfied by materials such as coated steel, but also by some high-strength brasses. In practice, either sliding shoes made from steel, onto the running surface of which a bronze with a high lead content has been cast, or sliding shoes which consist entirely of high-strength brasses are often used.
Steel slippers have high strength, depending on the grade of steel. They represent a preferred material for the base body. The poor tribological properties can be improved by casting cast bronzes with a high lead content onto the running surface. One drawback in this context is the cast structure with a low strength of the sliding layer which is formed by the casting-on process.
Corresponding slippers are described, for example, in document GB 1 355 325. The document discloses an axial piston pump or motor which comprises a slipper with friction-reducing layers at the contact surfaces between slippers and ball of the joint. These friction-reduced layers are metallically joined to the slippers, for example by friction welding, plating or by means of powder metallurgy. Intermediate layers of molybdenum may also be used.
Slippers for hydrostatic piston engines are also known from document DE-A 2 025 169; these slippers are supposed to be less susceptible to major temperature fluctuations in operation and to have good sliding properties. For this purpose, a layer of the same material as the slide plate is arranged fixedly on a surface of a slipper which faces the slide plate, so that disadvantageous convex bowing of the slide plate which otherwise occurs as a result of a corresponding arrangement of materials with different expansion coefficients can be substantially avoided.